kv-builder: Builder.add() mishandles filenames with equals ('=') signs

[arusso]

Problem

When Builder.add() is called, it is splitting the values on = to look for key/value pairs. This doesn't take into account that = is a valid character for a filename, and will mishandle files containing that character. An example of such a case can be seen in vault#12290

Expected Result

When Builder.add(string) encounters a raw value such as @cn=foo,o=example.com.json then it should read k/v from file cn=foo,o=example.com.json and not set {"@cn": "foo,o=example.com.json"}